Uninstall application proxy factory before exit

This should help with segfaults on exit.
Fixes #3657

qutebrowser/app.py

@@ -772,6 +772,8 @@ class Quitter:
                         pre_text="Error while saving {}".format(key))
         # Disable storage so removing tempdir will work
         websettings.shutdown()
+        # Disable application proxy factory to fix segfaults with Qt 5.10.1
+        proxy.shutdown()
         # Re-enable faulthandler to stdout, then remove crash log
         log.destroy.debug("Deactivating crash log...")
         objreg.get('crash-handler').destroy_crashlogfile()

qutebrowser/browser/network/proxy.py

@@ -34,6 +34,10 @@ def init():
     QNetworkProxyFactory.setApplicationProxyFactory(proxy_factory)
 
 
+def shutdown():
+    QNetworkProxyFactory.setApplicationProxyFactory(None)
+
+
 class ProxyFactory(QNetworkProxyFactory):
 
     """Factory for proxies to be used by qutebrowser."""